1. Field of the Invention
The present invention relates to a resonator having an electrode formed on a dielectric substrate, a filter, an oscillator, a duplexer, and a communication apparatus employing these devices.
2. Description of the Related Art
Resonators formed using a dielectric substrate and designed to exhibit resonance in the frequency band of microwaves or millimeter waves include a resonator realized with a slot line.
In a conventional slot-line resonator, one resonator is realized with a straight half-wave slot line. Such a resonator realized with a slot line is structured to have an electrode continuously formed around a slot line, and can therefore confine electromagnetic energy in the slot line with high efficiency. When the resonator is included as a module in a high-frequency circuit, it hardly interferes with any other circuit. This is advantageous.
FIG. 18A and FIG. 18B show an example of a half-wave slot-line resonator having both ends thereof short-circuited. In FIG. 18A, an electrode 2 having a slot 3 bored as part thereof is formed on the upper side of a dielectric substrate 1. FIG. 18B shows the distribution of the electromagnetic field on the slot-line resonator. In FIG. 18B, solid lines denote the electric field and dashed lines denote the magnetic field.
The efficiency of the resonator realized with a slot line in confining the electromagnetic field depends on the width of the slot. In other words, the larger the width of the slot 3 (slot line), the wider the spread of the electromagnetic field in the slot-line resonator.
The foregoing phenomenon will be interpreted below from a physical viewpoint.
For example, the electric field distribution in a slotted section is as shown in FIG. 19A. When the electric field distribution is expressed as an equivalent circuit, the equivalent circuit is like that shown in FIG. 19B or FIG. 19C. FIG. 19B shows an equivalent circuit for a large-width slot, while FIG. 19C shows an equivalent circuit for a small-width slot. In the equivalent circuit, if the ratio of an electrostatic capacitance C2 (C2′) or C3 (C3′) to the total electrostatic capacitance is large, or in other words, if the electrostatic capacitance C2 (C2′) or C3 (C3′) contributes greatly to the total electrostatic capacitance, the spread of the electromagnetic field is thought to be wide. In contrast, if the ratio is small or if the electrostatic capacitance C2 (C2′) or C3 (C3′) contributes little, the degree of concentration of the electromagnetic field in the slot is thought to be high.
Assuming that the lengths of electric lines of force drawn to pass through points at which the electrostatic capacitances C1 (C1′), C2 (C2′), and C3 (C3′) are detected are w1 (w1′), w2 (w2′), and w3 (w3′), respectively, the electrostatic capacitances are inversely proportional to the lengths of the electric lines of force. The lengths of the electric lines of force drawn to pass through the points at which the electrostatic capacitances are detected are assumed to change from those shown in FIG. 19B to those shown in FIG. 19C. This signifies that the width of the slot is decreased by a length Δw. In this case, the following relationships are obtained:w1′=w1−Δww2′=w2−Δww3′=w3−ΔwIn addition, a relationship of w1<w2<w3 holds. Among the changes from the capacitances C1, C2, and C3 to the capacitances C1′, C2′, and C3′, the change from C1 to C1′ is the largest. Namely, when the width of the slot is small, the electrostatic capacitance C1 (C1′) contributes most greatly to the total electrostatic capacitance. This means that a smaller width of a slot or a slot line leads to a higher degree of concentration of the electromagnetic field.
Therefore, for improving the efficiency of a slot-line resonator in confining the electromagnetic field, the width of a slot or a slot line should be decreased. A high-frequency circuit module is assumed to be composed of a slot-line resonator and another conductive line which are formed using a dielectric substrate. In this case, once the efficiency of the slot-line resonator in confining the electromagnetic field is improved, even if the distance from the slot-line resonator to the conductive line is decreased, undesirable coupling will hardly occur. The high-frequency circuit module can therefore be designed compactly.
When the width of a slot line in a slot-line resonator is decreased, the degree of current concentration at the edges of an electrode increases. Consequently, the edge effect becomes significant and conductor loss increases. The unloaded Q-factor (Qo) exhibited by the resonator decreases. Therefore, if the resonator is employed in a filter or the like, a new problem such as increased insertion loss will occur.